The field of the present invention is reversible hydraulic motor-pump units. More particularly, the present invention relates to power transfer units wherein two reversible hydraulic motor-pump units are coupled for torque transfer therebetween. Each one of the motor-pump units is associated with a separate hydraulic system having its own main high-pressure pump and fluid reservoir. By means of the power transfer unit, hydraulic power may be borrowed from one system for conversion into mechanical power by one of the motor-pump units, and then converted by the other motor-pump unit into hydraulic power which is supplied to the other of the two hydraulic systems.
It is conventional in modern aircraft to provide a plurality of separate hydraulic systems by which the various control functions of the aircraft may be performed. For example, the hydraulic systems of the aircraft may be used to move and selectively position control surfaces such as the slats or flaps of the wing, and to raise and lower the aircraft landing gear. In order to improve the level of flight safety, the hydraulic systems may be in redundant relationship with respect to performing some control functions. In order to provide such plural and partially redundant hydraulic systems while also minimizing the weight required for such systems, it is common to provide hydraulic power transfer units between the plural systems. These conventional hydraulic power transfer units provide for borrowing of hydraulic power from one system in order to meet a need in a coupled system which is beyond the supply capability of the primary high-pressure pump of that system which is borrowing power. Additionally, it is necessary that the hydraulic power transfer units prevent transfer of fluid between the coupled systems such that a failure of one system does not incapacitate a coupled system.
However, it is recognized in the field that conventional hydraulic power transfer units have several shortcomings. Among the shortcomings is a tendency for conventional units to operate too frequently. That is, a relatively low level of hydraulic pressure differential between two coupled hydraulic systems will result in conventional power transfer units operating in order to minimize the pressure differential between the coupled systems. Such overly frequent operation results in increased wear and shortened service life for conventional power transfer units. Another recognized shortcoming of conventional power transfer units is the possibility of failure of one portion of the power transfer unit resulting in failure of both of the coupled systems due to fluid leakage between the two systems.
Those conventional power transfer units which provide for bi-directional transfer of power between coupled hydraulic systems have in many cases also employed relatively complex electro-hydraulic control systems. Such complexity is undesirable because it provides additional failure modes for the power transfer unit. The necessity of providing electrical power to such units is also a disadvantage.